基于光谱呼吸自相似放大产生高质量飞秒激光脉冲的研究

High quality femtosecond laser pulses with high average power, short pulse duration and low noise have important applications in a number of research on frontier scientific and space technology, such as optical frequency combs, coherent pulse synthesis, spacecraft networking and formation flying. Yb-doped fiber self-similar amplifiers feature significant advantages in terms of generating such high quality femtosecond laser pulses. However, suffering from the finite gain bandwidth at high amplifier gains, the stimulated Raman scattering (SRS) and amplified spontaneous emission (ASE) disturbances in a long gain fiber, current performance of self-similar amplifiers are strongly limited. In this project, a novel spectral-breathing self-similar amplification method for high quality femtosecond laser pulse generation is proposed. By use of the nonlinear spectral compression and broadening, the gain-shaping disturbance can be avoided to accelerate self-similar pulse evolution in a high-gain short fiber amplifier, reduce the influence of SRS and ASE, increase the output pulse spectral width, and suppress the intensity and phase noise coupling. Through numerical simulations and systematic experiments, the dynamics and noise coupling physical mechanisms of spectral-breathing self-similar amplification will be explored. On this basis, guidelines for breaking the gain bandwidth and SRS limitations on the output pulse duration and power from self-similar amplifiers, as well as suppressing the noise during amplification will be achieved, generating high quality femtosecond laser pulses with high average power, short pulse duration and low noise.

高平均功率、窄脉冲宽度、低噪声的高质量飞秒激光在光学频率梳、相干脉冲合成、航天器组网和编队飞行等众多前沿科学及空间技术研究中均具有重要的应用价值。掺Yb光纤自相似放大技术在获得高质量飞秒激光方面具有突出优势，但高增益放大器的有限增益带宽以及长增益光纤引入的受激拉曼散射和自发辐射放大等扰动，却为此设置了障碍。本项目提出一种基于光谱呼吸自相似放大获得高质量飞秒激光的新方法。利用非线性光谱压缩及展宽，有效避免增益整形扰动，加速脉冲在高增益的短光纤放大器中的自相似演化，降低受激拉曼散射和自发辐射放大的影响，同时提高自相似脉冲光谱宽度，减小强度与相位噪声耦合。通过数值模拟结合系统实验，深入研究光谱呼吸自相似放大动力学过程及其噪声耦合物理机制，探索突破增益带宽和受激拉曼散射对自相似放大输出脉冲宽度和功率的限制，并抑制噪声耦合的技术途径，实现高平均功率、窄脉冲宽度、低噪声的高质量飞秒激光输出。

本项目针对光学频率梳、相干脉冲合成、航天器组网和编队飞行等前沿研究领域对高平均功率、窄脉冲宽度、低噪声的高质量飞秒激光的需求，开展基于光谱呼吸自相似放大获得高质量飞秒激光的研究。项目执行过程中，项目组建立了包含增益整形效应、受激拉曼散射扰动和自发辐射放大噪声的掺Yb光纤非线性放大数值模型，通过数值仿真定量分析了光纤长度、入射脉冲中心波长、能量、宽度、啁啾、形状等参数对光谱呼吸自相似放大过程和噪声特性的影响。实验建立了基于脉冲预优化装置的光谱呼吸自相似放大研究平台，系统研究了光谱呼吸自相似放大动力学过程及其噪声耦合物理机制，实现了高平均功率、窄脉冲宽度、低噪声的高质量飞秒激光输出。在本项目资助下，项目组取得了一系列创新性成果。首先，系统分析了掺Yb光纤放大器的增益整形作用，掌握了增益谱线随泵浦功率和增益光纤长度的演化曲线。其次，开展了强增益扰动下的自相似放大演化理论和实验研究，证实了在增益整形作用下，负啁啾脉冲放大的光谱呼吸行为以及三角形脉冲的高自相似演化效率。第三，利用光谱呼吸自相似放大演化产生低噪声变换极限飞秒激光脉冲，在2m长高增益掺Yb光纤放大器中获得了平均功率32 W、脉宽36 fs、强度噪声0.029% (1 kHz–5 MHz) 的高质量变换极限飞秒激光脉冲输出，证实了光谱呼吸自相似行为对增益整形扰动、泵浦抖动和噪声积累的抑制能力。第四，利用泵浦波长优化实现了飞秒激光放大器自相似演化加速和噪声抑制。第五，将光谱滤波机制引入到锁模激光器腔内，提出了一种无需色散管理的低时间抖动光纤飞秒激光器，为产生高能量低时间抖动飞秒激光提供了简单有效的方法。项目执行期间，发表SCI论文5篇，国际会议论文4篇，申请国家发明专利3项。